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Chapter: 11th 12th std standard Class Physics sciense Higher secondary school College Notes

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RADAR: Principle, Applications, Transmission and reception of radar

RADAR: Principle, Applications, Transmission and reception of radar
The term RADAR is an acronym for RAdio Detection And Ranging. It is a system which uses radio waves to detect and to fix the position of targets at a distance.

RADAR

 

The term RADAR is an acronym for RAdio Detection And Ranging. It is a system which uses radio waves to detect and to fix the position of targets at a distance.

 

Principle of radar

 

Radar works on the principle of 'radio echoes'. The transmitter in a radar, radiates the high power electrical pulses into space. When these pulses are incident on any distant target such as a mountain, ship or aircraft, they get scattered in all directions. The transmitter antenna receives a part of the scattered energy. This transmitter antenna also acts as receiving antenna for the receiving pulse. The pulse travels with the speed of light 3 108 ms-1. In other words, these pulses cover a distance of 300 metres for every micro second. Hence by measuring the time taken by the pulse to reach the target and back to the transmitter, the range or distance of the target can be easily determined. To locate the direction of the target, directional antennas are used.

 

Transmission and reception of radar

 

The block diagram of a simple radar system is shown in Fig  This block diagram indicates that the radar system consists of both the transmitting and the receiving system.

 

The transmitting system consists of a transmitter and a pulser. The receiving system consists of a receiver and an indicator. In most of the cases, a single antenna is used for both transmission and reception and this is achieved with the use of TR switch (Transmitter Receiver Switch). This switching arrangement is called as 'duplexer'. This connects the antenna to the transmitter during transmission and to the receiver during reception. Moreover, this switch isolates the sensitive receiver from the damaging effects of the high power transmitter.

 

The transmitter is essentially a high power magnetron oscillator which generates high power pulses. This transmitter is turned on and off with a periodic pulse from the pulser. Thus the transmitter generates periodic pulses of very short duration. These short pulses are fed to the antenna which radiates them into the space. The antenna is highly directional.

If the transmitted pulse hits any target, a weak echo signal returns to the same antenna. But, now the TR switch puts the antenna in contact with the receiver. This echo signal is amplified and demodulated by the superhet receiver. The sensitivity of the receiver is very high. The detected output is sent to the indicator. The indicator is a cathode ray tube. The CRT displays the original transmitted pulse as well as the detected echo pulse along a horizontal base line. The synchronising pulse generated by the 'timer' is supplied to both transmitting and receiving systems. So, the indicator records the transmitted pulse as well as the returning pulse simultaneously. The returning echo pulse appears slightly displaced from the transmitted pulse and this displacement is a measure of the range of the target.

 

 

Some applications of radar

 

1.     Air and sea navigation is made entirely safe, with radar installations. High flying planes and ship at sea, can get detailed reports of mountains, ice bergs, rivers, lakes, shore lines etc., which they can avoid.

2.     Radar systems are used for the safe landing of air crafts. On approaching the air field, the pilot is guided by signals from a radar set, so that it flies along the line of the runway and lands safely, whatever be the visibility.

3.     Rain drops may reflect suitable radar signals and thus enable meteorologists to measure the distance of the clouds, with great accuracy for forecasting.

4.     The pulses can be used for discovering the position of buried metals, oils and ores.

 


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